1. Field of the Invention
The invention in general relates to audio signaling devices having piezoelectric transducers which produce a sound signal, and more particularly to such a signaling device in which the start-up voltage may be preselected.
2. Description of the Prior Art
Audio signaling devices are widely used to signal the change of a condition or as a warning or alarm device in appliances, computers, detectors, specialized electronic devices, production machines, and many other applications. With the proliferation of the various types of electronic and other systems which employ such signaling devices, it has become important that such signaling devices be able to operate within many different voltage environments. In order to generate satisfactory and measurable sound output, sound devices of this type are specified over a voltage range where the low voltage of the range is higher than the start-up voltage. A signaling device that will not operate at all in one system that cannot produce a voltage high enough to drive it, may operate continuously in another system that has an "off-voltage" that is higher than the start-up voltage for the device. For example, a typical piezoelectric signaling device may have a start-up voltage of 0.6V, which start-up voltage is not acceptable for applications in some types of logic circuitry where the off-voltages are typically higher than 0.6V or in situations where the noise on supply lines can be higher than 0.6 V. Thus it would be highly desirable to have a piezoelectric signaling device in which the start-up voltage can be preselected to be any one of a number of different voltages. In addition, with the systematic and steady miniaturization and decrease in cost of the electronic systems which the signaling device is made a part of, it is important that the means of preselecting the starting voltage be small and inexpensive so that it does not add significantly to the overall size and cost of the signaling device.